The present invention relates generally to Variable Air Volume (VAV) Heating, Ventilating and Air Conditioning (HVAC) systems and particularly to the control of individual VAV terminal units served by a VAV system.
VAV systems primarily control the space temperature by varying the volume of supply air rather than the temperature of the supply air. The interior zones of most large buildings normally require only cooling because of occupancy and lighting loads. In a typical interior zone application, VAV air terminal units serve these zones and operate under thermostatic control to vary the airflow of cooling air to the individual spaces to maintain the required temperature. In some interior zone applications and in other applications the VAV terminal units may also include provisions for heating coils. VAV systems offer economy of installation and operation and have become the system of choice for many building occupancy types. A variety of air terminal units are available. For example, a single duct throttling type air terminal unit may be used with a space sensor that resets the setpoint of an air flow controller located at the air terminal unit to vary the volume of conditioned air to the space as required.
The availability of low cost digital electronics allows building managers to benefit from building control systems that use airflow sensors that are connected to distributed intelligence networks. This advance presents some new challenges, e.g., the processing power available to the sensor is limited by a local inexpensive processor, and the airflow sensor itself must be low cost.
Pressure-independent VAV systems require a calculation to convert the sensed value of the pressure, obtained via a flow pickup device which is stored internally as a voltage, to a value known as a flow velocity or air flow and is typically expressed in feet per minute (fpm). The representation of the transfer function between air pressure and volts in a pressure transducer is typically linear, but the conversion from air pressure to air velocity is a non-linear relationship more accurately described by the equation:
Airflow={square root over (Airpressure)}xc2x7K
where airflow is the airflow across the pressure transducer and the air pressure is the corresponding air pressure derived from the airflow. K is a constant for each flow sensor type that may ad just slightly with velocity.
This calculation of flow velocity must be performed frequently, for example, the calculation may typically be needed every second in order to ensure sufficient information required for timely airflow control. The calculation of airflow velocity requires both time and resource intensive processes to multiply and divide stored variables to obtain the required values. Typically, in a low cost processor, the time required to do multiply and divide operations on a value is very expensive, so for a given task in a typical process, the amount of operations allowed is limited to one or two calculations, which is a severe processing power calculation restriction.
An alternative to the frequent calculation of airflow values from measured air pressure values is to include a linearization table or equation in the airflow controller. The linearization table relates a specific number of air pressure values to a corresponding number of airflow values and can be used to construct a curve of airflow as a function of air pressure. A measured air pressure is then related to an airflow by using the curve. A significant number of calculations by the processor are required to relate a specific measured air pressure to a corresponding airflow.
There is a need to avoid additional runtime error limit checks which are expensive in terms of time and resources. There is a need to prevent microprocessor overflow and underflow.
In addition to the calculations necessary to determine airflow, the processor has a number of other tasks. For example., analog to digital conversion, providing frequent commands to a damper actuator, i/o communications, and network communications.
Large quantities of VAV terminal units and their associated controls are typically required in a building. Therefore it is important that the controls and installation techniques used be as cost effective as possible.
Thus, a need exists for a low-cost high-performance VAV controller that provides for accurate error and bounds checking within the processing power and resources of the local inexpensive processor.
The present invention solves these and other needs by providing a method for controlling a VAV air terminal unit that includes an airflow sensor for providing an airflow sensor signal to an airflow controller, a stored relationship between the airflow signal and an airflow value, a damper for varying airflow, and an actuator for positioning the damper in response to an airflow controller signal. The method includes energizing the controller by applying electrical power, determining either a minimum allowed air flow value based on the stored relationship, or determining a maximum allowed airflow value based on the stored relationship and periodically positioning the damper in response to the airflow controller signal, to provide an airflow value within a range that is limited by either the minimum allowed airflow value or the maximum allowed airflow value.